The disclosures of Japanese Patent Application Nos. 2000-090755 filed on Mar. 27, 2000, 2000-093023 filed on Mar. 28, 2000 and 2000-221877 filed on July 24, including the specifications, drawings and abstracts are incorporated herein by reference in their entirety.
1. Field of the Invention
The invention relates to an exhaust gas purifier for an internal combustion engine and method for purifying the exhaust gas.
2. Description of Related Art
Particulates mainly composed of soot are included in the exhaust gas in an internal combustion engine, especially a diesel engine. Because particulates are noxious, it has been proposed to dispose a filter, which collects particulates before they are discharged into the atmosphere, in an exhaust system of the engine. In order to prevent an increase in the exhaust gas resistance due to the clogging, such a filter is required to burn out the particulates collected therein.
During such regeneration of the filter, particulates burn through ignition as soon as they reach about 600xc2x0 C. However, since the temperature of the exhaust gas in a diesel engine is usually much lower than 600xc2x0 C., some measures are usually required for the purpose of heating the filter itself. Japanese Published Patent No. HEI 7-106290 discloses that the particulates on a filter continuously burn out approximately at 400xc2x0 C., namely, at the temperature of exhaust gas during normal operation of a diesel engine if a platinum-family metal and an alkaline earth metal oxide are carried on the filter.
However, the use of this filter does not guarantee that the temperature of exhaust gas is always approximately equal to 400xc2x0 C. In some operation states, a great amount of particulates may be discharged from the diesel engine, and the particulates that have not burnt out during each cycle may be gradually stacked on the filter.
If a certain amount of particulates are stacked on this filter, the capability to burn out the particulates deteriorates substantially to the extent of disabling the filter from being regenerated. Thus, even if such a filter is simply disposed in an exhaust system of an engine, the filter may get clogged soon.
It is an object of the invention to provide an exhaust gas purifier for an internal combustion engine and method for purifying the exhaust gas, which are capable of preventing the clogging of a particulate filter through reversal of upstream and downstream sides thereof with a simple structure and reducing the amount of particulates discharged into the atmosphere at the time of the reversal.
An exhaust gas purifier for an internal combustion engine according to a first aspect of the invention comprises a particulate filter disposed in an exhaust system of the engine to collect and oxidize particulates and a reversal device for reversing upstream and downstream sides of the particulate filter. The particulate filter has a collecting wall for collecting particulates. The collecting wall has first and second collecting surfaces. The first and second collecting surfaces of the collecting wall are used alternately to collect particulates through reversal of the upstream and downstream sides of the particulate filter by the reversal device. The reversal device has a valve body and reverses the upstream and downstream sides of the particulate filter by switching the valve body from one position to the other position. At least a portion of exhaust gas bypasses the particulate filter while the valve body is being switched from one position to the other position. The reversal device reverses the upstream and downstream sides of the particulate filter if the amount of particulates discharged from a combustion chamber of the engine has become equal to or smaller than a set discharge amount.
In some operation states, a certain amount of particulates remain on the first collecting surface of the collecting wall due to the insufficient oxidation of the particulates. However, according to the aforementioned construction, since the reversal device reverses the upstream and downstream sides of the particulate filter, no more particulates are stacked on the first collecting surface of the collecting wall and the residual particulates are gradually removed through oxidation. At the same time, the second collecting surface of the collecting wall starts collecting and oxidizing particulates. Thus, since the first and second collecting surfaces of the collecting wall are used alternately due to the reversal of the upstream and downstream sides of the particulate filter, the clogging of the particulate filter can be prevented. Furthermore, since the valve body is switched to reverse the upstream and downstream sides of the particulate filter when the amount of particulates discharged from the combustion chamber of the engine is equal to or smaller than the set discharge amount, there is no possibility that a great amount of particulates be included in exhaust gas. Even if a portion of exhaust gas has bypassed the particulate filter during the switching of the valve body, it is possible to reduce the amount of particulates discharged into the atmosphere.
In the aforementioned aspect, an active oxygen discharging agent may be carried on the collecting wall so that active oxygen discharged from the active oxygen discharging agent oxidizes particulates.
In the aforementioned aspect, the active oxygen discharging agent may absorb and retain oxygen if there is an excessive amount of oxygen around, and discharge the retained oxygen in the form of active oxygen if the concentration of ambient oxygen decreases.
In the aforementioned aspect, it may be determined that the discharge amount of particulates has become equal to or smaller than the set discharge amount as soon as it is judged that the engine is being decelerated, and the upstream and downstream sides of the particulate filter may be reversed.
In the aforementioned aspect, it may be judged upon detection of depression of a brake pedal that the engine is being decelerated.
In the aforementioned aspect, it may be judged upon detection of a decrease in the depression amount of an accelerator pedal that the engine is being decelerated.
In the aforementioned aspect, the reversal device may not reverse the upstream and downstream sides of the particulate filter even if the discharge amount of particulates has become equal to or smaller than the set discharge amount unless a set period has elapsed or a set running distance has been covered after the reversal of the upstream and downstream sides of the particulate filter.
In the aforementioned aspect, the reversal device may increase the switching speed of the valve body if it is judged during the switching of the valve body that the engine is being accelerated.
In the aforementioned aspect, it may be determined that the discharge amount of particulates has become equal to or smaller than the set discharge amount as soon as it is judged that the fuel injection amount is equal to or smaller than a set injection amount, and the upstream and downstream sides of the particulate filter may be reversed.
In the aforementioned aspect, it may be determined that the discharge amount of particulates has become equal to or smaller than the set discharge amount as soon as it is judged that the depression amount of an accelerator pedal is equal to or smaller than a set depression amount, and the upstream and downstream sides of the particulate filter may be reversed.
In the aforementioned aspect, it may be determined that the discharge amount of particulates has become equal to or smaller than the set discharge amount as soon as it is judged that a clutch pedal has been depressed, and the upstream and downstream sides of the particulate filter may be reversed.
An exhaust gas purifier for an internal combustion engine according to a second aspect of the invention comprises a particulate filter disposed in an exhaust system of the engine to collect and oxidize particulates and a reversal device for reversing upstream and downstream sides of the particulate filter. The particulate filter has a collecting wall for collecting particulates. The collecting wall has first and second collecting surfaces. The first and second collecting surfaces of the collecting wall are used alternately to collect particulates through reversal of the upstream and downstream sides of the particulate filter by the reversal device. The reversal device has a valve body and reverses the upstream and downstream sides of the particulate filter by switching the valve body from one position to the other position. At least a portion of exhaust gas bypasses the particulate filter while the valve body is being switched from one position to the other position. The reversal device reverses the upstream and downstream sides of the particulate filter at a switching speed corresponding to an operation state of the engine.
As described above, the aforementioned construction makes it possible to prevent the clogging of the particulate filter through the reversal of the upstream and downstream sides of the particulate filter. In addition, the valve body is switched to reverse the upstream and downstream sides of the particulate filter at a switching speed corresponding to an operation state of the engine. Therefore, if the switching speed is reduced when almost no particulates are included in low-temperature exhaust gas, for example, when the engine is being decelerated, it is possible to suppress a decrease in the temperature of the particulate filter without discharging particulates into the atmosphere. If the switching speed is increased when particulates are included in exhaust gas, it is possible to reduce the amount of particulates discharged into the atmosphere.
In the aforementioned second aspect, an active oxygen discharging agent may be carried on the collecting wall so that active oxygen discharged from the active oxygen discharging agent oxidizes particulates.
In the aforementioned second aspect, the active oxygen discharging agent may absorb and retain oxygen if there is an excessive amount of oxygen around, and discharge the retained oxygen in the form of active oxygen if the concentration of ambient oxygen decreases.